Various sizes and shapes of plastic containers have been used to contain various liquids (e.g. edible oils, liquid seasonings, beverages, alcohols, detergents for kitchen, detergents for clothes, shampoos, hair dressing agents, liquid soaps, rubbing alcohols, oils for automobiles, detergents for automobiles, agricultural chemicals, insecticides, and herbicides) to transport, to display, to purchase, to store, and to use the liquid. These plastic containers are typically produced by blow molding or the like as plastic containers having a single layer or a plurality of layers using a resin, such as polyethylene, polypropylene, polyester, or polyamide.
Furthermore, these plastic containers are provided with labels with trade names and other information to identify the contents thereof. In many cases, such a label is provided on a plastic container after the plastic container is formed, by providing a pressure-sensitive adhesive agent layer on a paper material or by using a heat-shrinkable film; however, the label can be provided on the container at the same time of the formation of the plastic container.
A method in which a label is placed in a mold, and the label is provided on a plastic container at the same time as the molding of the plastic container in the mold is typically referred to as an in-mold label process. Since the in-mold label process does not require adhering of the label after molding the container or storing the molded product in the middle of the process, it is advantageous in that labor saving, reduction in space required to store products in the middle of the process, and immediate shipping are made possible. As the label used in the in-mold label process (in-mold molding label), typically, a thermoplastic resin film having a laminate structure, in which a heat seal layer having thermal adhesiveness is formed on a base layer containing a thermoplastic resin as a main component, is used. In this thermoplastic resin film, the face located on the opposite side of the base layer of the heat seal layer (back surface) is used as a heat seal surface that is thermally adhered to a plastic container, and the face located on the opposite side of the heat seal layer of the base layer (front surface) is used as a printing surface on which information, such as characters and images, is printed.
In the in-mold label process, when it is difficult for air trapped in between resin and an in-mold label before molding to escape, the air may remain in between the label and the plastic container after the molding, thereby reducing the adhesive strength of the label and/or causing failure in appearance, which is a so-called blister.
Therefore, air flow paths are formed by making recessions and protrusions on a surface on the side to which the plastic container is adhered (heat seal surface) of the in-mold label.
For example, Patent Document 1 discloses an embossing pattern that does not cause blisters due to high adhesive strength of the label as a result of comparing and studying embossing patterns of forward gravure and reverse gravure as well as the dot shape and the number of lines of embossing.
Furthermore, Patent Document 2 discloses that a structure in which rows of protrusions are arranged in a particular pattern was obtained by a method including embossing in a laminating step of a heat seal layer as a result of comparing parameters of publicly known embossing patterns.